Generally, the level of acceptable emissions from vehicular internal combustion engines is regulated by legislation. Such levels are being tightened in the years to come, and so the challenge for vehicle manufacturers (original equipment manufacturers or OEMs) is how to meet them. Amongst the legislated exhaust gas components are particulate matter (PM), nitrogen oxides (NOx), carbon monoxide (CO) and hydrocarbons (HC). A widely adopted measure to meet legislated levels on PM is the particulate or soot filter. Broadly, such filters increase the residence time of PM in an exhaust system to enable it to be destroyed and can include ceramic wall-flow filters or wire mesh filters.
Typically, a wall-flow filter is in the form of a honeycomb. The honeycomb has an inlet end and an outlet end, and a plurality of cells extending from the inlet end to the outlet end, the cells having porous walls wherein part of the total number of cells at the inlet end are plugged, e.g. to a depth of about 5 to 20 mm, along a portion of their lengths, and the remaining part of the cells that are open at the inlet end are plugged at the outlet end along a portion of their lengths, so that a flowing exhaust gas stream passing through the cells of the honeycomb from the inlet end flows into the open cells, through the cell walls, and out of the filter through the open cells at the outlet end. A composition for plugging the cells is described in U.S. Pat. No. 4,329,162 (incorporated herein by reference). A typical arrangement is to have every other cell on a given face plugged, as in a chequered pattern.
A problem associated with the use of particulate filters is how to destroy the PM collected from an exhaust gas throughout a lean burn engine cycle. Generally, diesel PM combusts in oxygen (O2) at above about 550° C. However, diesel exhaust gas temperatures, particularly in light-duty diesel engines, can be as low as 150° C. during certain phases of a drive cycle due, for example, to the increasingly heavy use of exhaust gas recirculation (EGR) to lower NOx emissions. If PM is allowed to build up, the back-pressure can increase thereby increasing the load on the engine. Increased engine load can lead to increased fuel consumption and, in a worst case, engine wear or destruction of the filter caused by uncontrolled combustion of large amounts of PM. Whilst increasing the engine load, e.g. through increased back-pressure due to PM build-up, can also increase the exhaust gas temperature sufficiently to combust the PM, such temperature increase can be insufficient reliably to keep the filter clear.
Light-duty diesel engines are defined in European legislation by European Directive 70/220/EEC, as amended by 93/59/EC and 98/69/EC. In the USA passenger vehicles, light light-duty trucks (LLDT), below 6000 lbs gross vehicle weight rating (GVWR) and heavy light-duty trucks (HLDT), above 6000 lbs are included in the light-duty diesel category. The exhaust gas temperatures emitted from light-duty diesel engines are generally lower than those of heavy-duty diesel engines (as defined by the relevant legislation).
It is known to catalyse particulate filters in order to lower the soot combustion temperature to facilitate regeneration of the filter passively by oxidation of PM under exhaust temperatures experienced during regular operation of the engine/vehicle, typically in the 300-400° C. range. In the absence of the catalyst, PM can be oxidized at appreciable rates at temperatures in excess of 500° C., which are rarely seen in diesel engines during real-life operation. Such catalysed filters are often called catalysed soot filters (or CSFs).
A common problem with passive filter regeneration is that driving conditions can prevent exhaust gas temperatures achieving even the lower temperatures facilitated by catalysing the filter frequently enough to reliably prevent PM from building up on the filter. Such driving conditions include extended periods of engine idling or slow urban driving and the problem is particularly acute for exhaust gas from light-duty diesel engines. One solution to this problem which has been adopted by OEMs is to use active techniques to regenerate the filter either at regular intervals or when a predetermined filter backpressure is detected in addition to passive regeneration. A typical arrangement in a light-duty diesel vehicle is to position a diesel oxidation catalyst (DOC) on a separate monolith upstream of the CSF and to regulate in-cylinder fuel combustion by various engine management techniques in order to introduce increased amounts of unburned fuel into the exhaust gas. The additional fuel is combusted on the DOC, increasing the temperature in the downstream CSF sufficiently to promote combustion of PM thereon.
A significant advance in treating PM was made with our discovery that diesel PM can be combusted in nitrogen dioxide (NO2) at up to 400° C. (see our EP-B-0341832 (incorporated herein by reference)). NO2 can be obtained by oxidising nitrogen monoxide (NO) in the exhaust gas over a suitable oxidation catalyst and reacted with PM on a downstream filter. This advance enables the PM to be destroyed within a normal exhaust gas temperature window for many diesel engines. We market devices incorporating this process as CRT®. However, whilst the process has been adopted successfully in heavy-duty diesel applications, there still remain difficulties with its use in certain lean burn internal combustion engines, particularly light-duty diesel engines. The recurring problem is low exhaust gas temperature, e.g. thermodynamic limitation on PM combustion in NO2 and the NO to NO2 equilibrium.
The process of absorbing NOx from a lean exhaust gas on a NOx absorbent such as barium to “store” it as the nitrate and release the stored NOx and reduce it to dinitrogen (N2) in exhaust gas containing less oxygen is known, e.g. from EP 0560991 (incorporated herein by reference). Typically, when this technology is used in practice, techniques are employed to assess the remaining capacity of the NOx absorbent and for controlling the engine to switch transiently and intermittently to running conditions producing exhaust gas having a lower O2 concentration relative to normal lean running conditions (i.e. enriched exhaust gas) in order to remove the stored NOx as dinitrogen (N2), thereby to regenerate the NOx absorbent.
We believe that the annotations to FIGS. 1-5 inclusive are self-explanatory. “NOx (1)” in the Figures is the first NOx absorbent; “NOx (2)” is the second NOx absorbent; and “CSF” is an acronym for catalysed soot filter.